Precise measurements play a vital role in all aspects of science, technology, and commerce. In principle, the fundamental limit of measurement precision is set by the laws of quantum mechanics. For optical sensing, this is known as the shot-noise limit, and is a consequence of the discrete nature of photons. This limit not only provides the most precise measurement possible with independent photons, but also is the essential stepping-stone for quantum-enhanced sensing that relies on uniquely quantum features such as squeezing and entanglement. A common strategy to approaching the shot-noise limit is to increase the intensity of the light, as the shot noise scales inversely with the intensity of light used for sensing. In practice however, the saturation of detectors sets a tight limit on the intensity of the detected light, beyond which the enhancement in the measurement precision by increasing the light intensity is reduced or even eliminated.

In this work, we show that the use of technique known as weak-value amplification, some real-world imperfections such as saturation can be overcome to approach the shot-noise limit. Weak-value amplification works by coupling the state of the light to be measured with another system or degree of freedom (in our experiment, the polarisation) which is then prepared and measured in specific quantum states. This has the effect of channelling information into particular aspects of a detector’s output signal, thereby allowing us to match them to the detector aspects that have the best specifications. Our experiments, performed at the University of Nanjing, used a CCD camera as the detector, and showed that shot-noise-limited measurements are possible with intensities more than an order of magnitude beyond where the camera saturates.

• The work appears on the cover of Physical Review Letters, volume 125, issue 8. https://journals.aps.org/prl/issues/125/8 
• For this work, Animesh Datta and his group were supported by an EPSRC Fellowship, the UK Quantum Enhanced Imaging hub, and the University of Warwick Global Partnership Fund.